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Network Working Group                                            H. Chen
Internet-Draft                                                  D. Cheng
Intended status: Standards Track                     Huawei Technologies
Expires: July 11, 2019                                            M. Toy
                                                                 Verizon
                                                                 Y. Yang
                                                                     IBM
                                                                 A. Wang
                                                           China Telecom
                                                                  X. Liu
                                                          Volta Networks
                                                                  Y. Fan
                                                            Casa Systems
                                                                  L. Liu
                                                         January 7, 2019


                         LS Flooding Reduction
                   draft-cc-lsr-flooding-reduction-01

Abstract

   This document proposes an approach to flood link states on a topology
   that is a subgraph of the complete topology per underline physical
   network, so that the amount of flooding traffic in the network is
   greatly reduced, and it would reduce convergence time with a more
   stable and optimized routing environment.  The approach can be
   applied to any network topology in a single area.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 11, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Flooding Topology . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Construct Flooding Topology . . . . . . . . . . . . . . .   6
     5.2.  Protect Flooding Topology Split . . . . . . . . . . . . .   7
   6.  Extensions to OSPF  . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Extensions for Operations . . . . . . . . . . . . . . . .   8
     6.2.  Extensions for Centralized Mode . . . . . . . . . . . . .   9
       6.2.1.  Message for Flooding Topology . . . . . . . . . . . .   9
       6.2.2.  Leaders Selection . . . . . . . . . . . . . . . . . .  17
   7.  Extensions to IS-IS . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Extensions for Operations . . . . . . . . . . . . . . . .  18
     7.2.  Extensions for Centralized Mode . . . . . . . . . . . . .  19
       7.2.1.  TLV for Flooding Topology . . . . . . . . . . . . . .  19
       7.2.2.  Leaders Selection . . . . . . . . . . . . . . . . . .  20
   8.  Flooding Behavior . . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Nodes Perform Flooding Reduction without Failure  . . . .  21
       8.1.1.  Receiving an LS . . . . . . . . . . . . . . . . . . .  21
       8.1.2.  Originating an LS . . . . . . . . . . . . . . . . . .  21
       8.1.3.  Establishing Adjacencies  . . . . . . . . . . . . . .  21
     8.2.  An Exception Case . . . . . . . . . . . . . . . . . . . .  22
       8.2.1.  Multiple Failures . . . . . . . . . . . . . . . . . .  22
       8.2.2.  Changes on Flooding Topology  . . . . . . . . . . . .  23
   9.  Operations on Flooding Reduction  . . . . . . . . . . . . . .  23



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     9.1.  Configuring Flooding Reduction  . . . . . . . . . . . . .  24
       9.1.1.  Configurations for Centralized Flooding Reduction . .  24
       9.1.2.  Configurations for Distributed Flooding Reduction . .  24
     9.2.  Migration to Flooding Reduction . . . . . . . . . . . . .  24
       9.2.1.  Migration to Centralized Flooding Reduction . . . . .  24
       9.2.2.  Migration to Distributed Flooding Reduction . . . . .  25
     9.3.  Roll Back to Normal Flooding  . . . . . . . . . . . . . .  25
     9.4.  Transfer from Distributed to Centralized Mode . . . . . .  26
     9.5.  Transfer from Centralized to Distributed Mode . . . . . .  27
     9.6.  Adding a New Node to Network  . . . . . . . . . . . . . .  27
   10. Manageability Considerations  . . . . . . . . . . . . . . . .  28
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.2.  OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . .  29
     12.3.  IS-IS  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Algorithms to Build Flooding Topology  . . . . . . .  32
     A.1.  Algorithms to Build Tree without Considering Others . . .  32
     A.2.  Algorithms to Build Tree Considering Others . . . . . . .  33
     A.3.  Connecting Leaves . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   For some networks such as dense Data Center (DC) networks, the
   existing Link State (LS) flooding mechanism is not efficient and may
   have some issues.  The extra LS flooding consumes network bandwidth.
   Processing the extra LS flooding, including receiving, buffering and
   decoding the extra LSs, wastes memory space and processor time.  This
   may cause scalability issues and affect the network convergence
   negatively.

   This document proposes an approach to minimize the amount of flooding
   traffic in the network.  Thus the workload for processing the extra
   LS flooding is decreased significantly.  This would improve the
   scalability, speed up the network convergence, stable and optimize
   the routing environment.

   This approach is also flexible.  It has multiple modes for
   computation of flooding topology.  Users can select a mode they
   prefer, and smoothly switch from one mode to another.  The approach
   is applicable to any network topology in a single area.  It is
   backward compatible.




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2.  Terminology

   Flooding Topology:
       A sub-graph or sub-network of a given (physical) network topology
       that has the same reachability to every node as the given network
       topology, through which link states are flooded.

   Critical link or interface on a flooding topology:
       A only link or interface among some nodes on the flooding
       topology.  When this link or interface goes down, the flooding
       topology will be split.

   Critical node on a flooding topology:
       A only node connecting some nodes on the flooding topology.  When
       this node goes down, the flooding topology will be split.

   Backup path:
       A path or a sequence of links, providing an alternative
       connection between the two end nodes of a link on the flooding
       topology or between the two end nodes of a path crossing a node
       on the flooding topology.  when a critical link goes down, the
       backup path for the link provides a connection to connect two
       parts of a split flooding topology.  When a critical node goes
       down, the backup paths for the paths crossing the node connect
       all the split parts of the floooding topology into one.

   Remaining Flooding Topology:
       A topology from a flooding topology by removing the failed links
       and nodes from the flooding topology.

   LSA:
       A Link State Advertisement in OSPF.

   LSP:
       A Link State Protocol Data Unit (PDU) in IS-IS.

   LS:
       A Link Sate, which is an LSA or LSP.

3.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].







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4.  Problem Statement

   OSPF and IS-IS deploy a so-called reliable flooding mechanism, where
   a node must transmit a received or self-originated LS to all its
   interfaces (except for the interface where an LS is received).  While
   this mechanism assures each LS being distributed to every node in an
   area or domain, the side-effect is that the mechanism often causes
   redundant LS, which in turn forces nodes to process identical LS more
   than once.  This results in the waste of link bandwidth and nodes'
   computing resources, and the delay of topology convergence.

   This becomes more serious in networks with large number of nodes and
   links, and in particular, higher degree of interconnection (e.g.,
   meshed topology, spine-leaf topology, etc.).  In some environments
   such as in data centers, the drawback of the existing flooding
   mechanism has already caused operational issues, including waves of
   flooding storms, choke of computing resources, slow convergence,
   oscillating topology changes, and instability of routing environment.

   One example is as shown in Figure 1, where Node 1, Node 2 and Node 3
   are interconnected in a mesh.  When Node 1 receives a new or updated
   LS on its interface I11, it by default would forward the LS to its
   interface Il2 and I13 towards Node 2 and Node 3, respectively, after
   processing.  Node 2 and Node 3 upon reception of the LS and after
   processing, would potentially flood the same LS over their respective
   interface I23 and I32 toward each other, which is obviously not
   necessary and at the cost of link bandwidth as well as both nodes'
   computing resource.

                                   |
                                   |I11
                                +--o---+
                                |Node 1|
                                +-o--o-+
                             I12 /    \ I13
                                /      \
                            I21/        \I31
                         +----o-+   I32+-o----+
                         |Node 2|------|Node 3|
                         +------+I23   +------+

                                 Figure 1

5.  Flooding Topology

   For a given network topology, a flooding topology is a sub-graph or
   sub-network of the given network topology that has the same
   reachability to every node as the given network topology.  Thus all



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   the nodes in the given network topology MUST be in the flooding
   topology.  All the nodes MUST be inter-connected directly or
   indirectly.  As a result, LS flooding will in most cases occur only
   on the flooding topology, that includes all nodes but a subset of
   links.  Note even though the flooding topology is a sub-graph of the
   original topology, any single LS MUST still be disseminated in the
   entire network.

5.1.  Construct Flooding Topology

   Many different flooding topologies can be constructed for a given
   network topology.  A chain connecting all the nodes in the given
   network topology is a flooding topology.  A circle connecting all the
   nodes is another flooding topology.  A tree connecting all the nodes
   is a flooding topology.  In addition, the tree plus the connections
   between some leaves of the tree and branch nodes of the tree is a
   flooding topology.

   The following parameters need to be considered for constructing a
   flooding topology:

   o  Number of links: The number of links on the flooding topology is a
      key factor for reducing the amount of LS flooding.  In general,
      the smaller the number of links, the less the amount of LS
      flooding.

   o  Diameter: The shortest distance between the two most distant nodes
      on the flooding topology (i.e., the diameter of the flooding
      topology) is a key factor for reducing the network convergence
      time.  The smaller the diameter, the less the convergence time.

   o  Redundancy: The redundancy of the flooding topology means a
      tolerance to the failures of some links and nodes on the flooding
      topology.  If the flooding topology is split by some failures, it
      is not tolerant to these failures.  In general, the larger the
      number of links on the flooding topology is, the more tolerant the
      flooding topology to failures.

   There are many different ways to construct a flooding topology for a
   given network topology.  A few of them are listed below:

   o  Centralized Mode: One node in the network builds a flooding
      topology and floods the flooding topology to all the other nodes
      in the network (Note: Flooding the flooding topology may increase
      the flooding.  The amount of traffic for flooding the flooding
      topology should be minimized.);





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   o  Distributed Mode: Each node in the network automatically
      calculates a flooding topology by using the same algorithm (No
      flooding for flooding topology);

   o  Static Mode: Links on the flooding topology are configured
      statically.

   Note that the flooding topology constructed by a node is dynamic in
   nature, that means when the base topology (the entire topology graph)
   changes, the flooding topology (the sub-graph) MUST be re-computed/
   re-constructed to ensure that any node that is reachable on the base
   topology MUST also be reachable on the flooding topology.

   For reference purpose, some algorithms that allow nodes to
   automatically compute flooding topology are elaborated in Appendix A.
   However, this document does not attempt to standardize how a flooding
   topology is established.

5.2.  Protect Flooding Topology Split

   It is hard to construct a flooding topology that reduces the amount
   of LS flooding greatly and is tolerant to multiple failures.  Without
   any protection against a flooding topology split when multiple
   failures on the flooding topology happen, we may have a slow
   convergence.  For example, in centralized mode, it takes some time
   for the leader to detect the failures through receiving the link
   states, compute a new flooding topology and flood the new flooding
   topology.  In addition, it takes some time for each of the other
   nodes to receive the new flooding topology (piece by piece), decode
   it and build it locally.  It is better to have some simple and fast
   methods for protecting the flooding topology split.  Thus the
   convergence is not slowed down.

   In one way, when two or more failures on the current flooding
   topology occur almost in the same time, each of the nodes within a
   given distance (such as 3 hops) to a failure point, floods the link
   state (LS) that it receives to all the links (except for the one from
   which the LS is received) until a new flooding topology is built.

   In another way, each node computes and maintains a small number of
   backup paths.  For a backup path for a link L on the flooding
   topology, a node N computes and maintains it only if the backup path
   goes through node N.  Node N stores the links (e.g., local link L1
   and L2) attached to it and on the backup path.  When link L fails and
   there are one or more other failures on the flooding topology, node N
   adds the links (e.g., L1 and L2) to the flooding topology temporarily
   until a new flooding topology is built.  Suppose that the two end
   nodes of link L is A and B, and A's ID is smaller than B's.  Node N



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   computes a path from A to B with minimum hops and whose links are not
   on the flooding topology.  This path is a backup path for link L.

6.  Extensions to OSPF

   The extensions to OSPF comprises two parts: one part is for
   operations on flooding reduction, the other is specially for
   centralized flooding reduction (or say flooding reduction in
   centralized mode).

6.1.  Extensions for Operations

   A new TLV is defined in OSPF RI LSA [RFC7770].  It contains
   instructions about flooding reduction, and is called Flooding
   Reduction Instruction TLV or Instruction TLV for short.  This TLV is
   originated from only one node and has the format below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      INST-TLV-Type (TBD1)     |          TLV-Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OP | MOD |   Algorithm   |    Reserved (MUST be zero)  |  NL |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Flooding Reduction Instruction TLV


   A OP field of three bits is defined in the TLV.  It may have a value
   of the followings.

   o  0x001 (R): Perform flooding Reduction, which instructs the nodes
      in an area to perform flooding reduction.

   o  0x010 (N): Roll back to Normal flooding, which instructs the nodes
      in an area to roll back to perform normal flooding.

   When any of the other values is received, it is ignored.

   A MOD field of three bits is defined in the TLV and may have a value
   of the followings.

   o  0x001 (C): Centralized Mode, which instructs: 1) the nodes in an
      area to select leaders (primary/designated leader, secondary/
      backup leader, and so on); 2) the primary leader to compute a
      flooding topology and flood it to all the other nodes in the area;



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      3) every node in the area to receive and use the flooding topology
      originated by the primary leader.

   o  0x010 (D): Distributed Mode, which instructs every node in an area
      to compute and use its own flooding topology.

   o  0x011 (S): Static Mode, which instructs every node in an area to
      use the flooding topology statically configured on the node.

   When any of the other values is received, it is ignored.

   An Algorithm field of eight bits is defined in the TLV to instruct
   the leader node in centralized mode or every node in distributed mode
   to use the algorithm indicated in this field for computing a flooding
   topology.

   A NL field of three bits is defined in the TLV, which indicates the
   number of leaders to be selected when Centralized Mode is used.  NL
   set to 2 means two leaders (a designated/primary leader and a backup/
   secondary leader) to be selected for an area, and NL set to 3 means
   three leaders to be selected.  When Centralized Mode is not used, The
   NL field is not valid.

   Some optional sub TLVs may be defined in the future, but none is
   defined now.

6.2.  Extensions for Centralized Mode

6.2.1.  Message for Flooding Topology

   A flooding topology can be represented by the links in the flooding
   topology.  For the links between a local node and its adjacent (or
   remote) nodes, we can encode the local node and its adjacent nodes.
   After all the links in the flooding topology are encoded, the encoded
   links can be flooded to every node in the network.  After receiving
   the encoded links, every node decodes the links and creates and/or
   updates the flooding topology.

   Every node orders the nodes by their node IDs (router IDs in OSPF,
   system IDs in IS-IS) in ascending order, and generates the same
   sequence of the nodes in the area.  The sequence of nodes have the
   index 0, 1, 2, and so on respectively.  Every node in the encoded
   links is represented by its index.








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6.2.1.1.  Links Encoding

   A local node can be encoded in two parts: encoded node index size
   indication (ENSI) of 4 bits and compact node index (CNI).  ENSI value
   plus 8 gives the size of compact node index.  For example, ENSI = 0
   indicates that the size of CNIs is 8 bits.  In the figure below,
   Local node LN1 is encoded as ENSI=0 using 4 bits and CNI=LN1's Index
   using 8 bits.  LN1 is encoded in 12 bits in total.

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |0 0 0 0|            ENSI (4 bits) [0 + 8 = 8 bits CNI]
    +-+-+-+-+-+-+-+-+
    |  LN1's Index  |    CNI  (8 bits)
    +-+-+-+-+-+-+-+-+

               Encoding for local node LN1


   The adjacent nodes can be encoded in two parts: Number of Nodes (NN)
   of 4 bits and compact node indexes (CNIs).  The size of CNIs is the
   same as the local node.  For example, local node LN1 has three
   adjacent nodes RN1, RN2 and RN3 in the following figure.

                                   o LN1
                                 / | \
                               /   |   \
                             /     |     \
                           o RN1   o RN2   o RN3

             Links from LN1 to its adjacent nodes RN1, RN2 and RN3


   These three adjacent nodes RN1, RN2 and RN3 are encoded below in 28
   bits (i.e., 3.5 bytes).
















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     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |0 0 1 1|           NN (4 bits)   [3 adjacent nodes]
    +-+-+-+-+-+-+-+-+
    |  RN1's Index  |   CNI (8 bits) for RN1
    +-+-+-+-+-+-+-+-+
    |  RN2's Index  |   CNI (8 bits) for RN2
    +-+-+-+-+-+-+-+-+
    |  RN3's Index  |   CNI (8 bits) for RN3
    +-+-+-+-+-+-+-+-+

         Encoding for three adjacent nodes RN1, RN2 and RN3


   The links between a local node and its adjacent (or remote) nodes can
   be encoded as the local node followed by the adjacent nodes.  For
   example, three links between local node LN1 and its three adjacent
   nodes RN1, RN2 and RN3 are encoded below in 40 bits (i.e., 5 bytes).

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+                              _
    |0 0 0 0|           ENSI (4 bits) [8 bits CNI]  |
    +-+-+-+-+-+-+-+-+                               } Encoding for
    |  LN1's Index  |   CNI (8 bits) for LN1       _| Local Node LN1
    +-+-+-+-+-+-+-+-+                              _
    |0 0 1 1|           NN (4 bits) [3 nodes]       |
    +-+-+-+-+-+-+-+-+                               | Encoding for
    |  RN1's Index  |   CNI (8 bits) for RN1        | 3 adjacent nodes
    +-+-+-+-+-+-+-+-+                               } RN1, RN2, RN3
    |  RN2's Index  |   CNI (8 bits) for RN2        | of LN1
    +-+-+-+-+-+-+-+-+                               |
    |  RN3's Index  |   CNI (8 bits) for RN3       _|
    +-+-+-+-+-+-+-+-+

           Links Encoding for links from LN1 to RN1, RN2 and RN3


   For a flooding topology computed by a leader of an area, it is
   represented by all the links on the flooding topology.  A Type-
   Length-Value (TLV) of the following format for the links encodings is
   included in an LSA to represent the flooding topology (FT).










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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      FTLK-TLV-Type (TBD2)     |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                         Flooding Topology Links TLV


   Note that a link between a local node LN and its adjacent node RN is
   encoded once and as a bi-directional link.  That is that if it is
   encoded in a Links Encoding from LN to RN, then the link from RN to
   LN is implied or assumed.

   For OSPFv2, an Opaque LSA of a new opaque type (TBD3) containing a
   Flooding Topology Links TLV is used to flood the flooding topology
   from the leader of an area to all the other nodes in the area.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   | LS Type = 10  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | FT-Type(TBD3) |                   Instance ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Flooding Topology Links TLV                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     OSPFv2: Flooding Topology Opaque LSA


   For OSPFv3, an area scope LSA of a new LSA function code (TBD4)
   containing a Flooding Topology Links TLV is used to flood the
   flooding topology from the leader of an area to all the other nodes
   in the area.





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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |1|0|1|       FT-LSA (TBD4)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Flooding Topology Links TLV                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         OSPFv3: Flooding Topology LSA


   The U-bit is set to 1, and the scope is set to 01 for area-scoping.

6.2.1.2.  Block Encoding

   Block encoding uses a single structure to encode a block (or part) of
   topology, which can be a block of links in a flooding topology.  It
   can also be all the links in the flooding topology.  It starts with a
   local node LN and its adjacent (or remote) nodes RNi (i = 1, 2, ...,
   n), and can be considered as an extension to the links encoding.

   The encoding of links between a local node and its adjacent nodes
   described in Section 6.2.1.1 is extended to include the links
   attached to the adjacent nodes.

   The encoding for the adjacent nodes is extended to include Extending
   Flags (E Flags for short) between the NN (Number of Nodes) field and
   the CNIs (Compact Node Indexes) for the adjacent nodes.  The length
   of the E Flags field is NN bits.  The following is an encoding of
   LN1's adjacent nodes RN1, RN2 and RN3 with E Flags of 3 bits, which
   is the value of the NN (the number of adjacent nodes).












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     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+                              _
    |0 0 1 1|           NN(4 bits)[3 adjacent nodes]|
    +-+-+-+-+                                       |
    |1 0 1|             E Flags [NN=3 bits]         | Encoding for
    +-+-+-+-+-+-+-+-+                               | 3 adjacent nodes
    |  RN1's Index  |   CNI (8 bits) for RN1        } (RN1, RN2, RN3)
    +-+-+-+-+-+-+-+-+                               | of LN1
    |  RN2's Index  |   CNI (8 bits) for RN2        | with E Flags
    +-+-+-+-+-+-+-+-+                               |
    |  RN3's Index  |   CNI (8 bits) for RN3       _|
    +-+-+-+-+-+-+-+-+

    Encoding of LN1's Adjacent Nodes RN1, RN2 and RN3 with E Flags


   There is a bit flag (called E flag) in the E Flags field for each
   adjacent node.  The first bit (i.e., the most significant bit) in the
   E Flags field is for the first adjacent node (e.g., RN1), the second
   bit is for the second adjacent node (e.g., RN2), and so on.  The E
   flag for an adjacent node RNi set to one indicates that the links
   attached to the adjacent node RNi are included below.  The E flag for
   an adjacent node RNi set to zero means that no links attached to the
   adjacent node RNi are included below.

   The links attached to the adjacent node RNi are represented by the
   RNi as a local node and the adjacent nodes of RNi.  The encoding for
   the adjacent nodes of RNi is the same as that for the adjacent nodes
   of a local node.  It consists of an NN field of 4 bits, E Flags field
   of NN bits, and CNIs for the adjacent nodes of RNi.

   The following is an example of a block encoding for a flooding
   topology (FT) block below.

                             o LN1
                           / | \
                         /   |   \
                       /     |     \
                     o RN1   o RN2   o RN3
                   /               /   \
                 /               /       \
               /               /           \
              o RN11          o RN31        o RN32

     FT Block from LN1 to RN1, RN2 and RN3, and to RN11, RN31 and RN32






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   It represents 6 links: 3 links between local node LN1 and its 3
   adjacent nodes RN1, RN2 and RN3; 1 link between RN1 as a local node
   and its 1 adjacent node RN11; and 2 links between RN3 as a local node
   and its 2 adjacent nodes RN31 and RN32.

   It starts with the encoding of the links between local node LN1 and 3
   adjacent nodes RN1, RN2 and RN3 of the local node LN1.  The encoding
   for the local node LN1 is the same as that for a local node described
   in Section 6.2.1.1.  The encoding for 3 adjacent nodes RN1, RN2 and
   RN3 of local node LN1 comprises an NN field of 4 bits with value of
   3, E Flags field of NN = 3 bits, and the indexes of adjacent nodes
   RN1, RN2 and RN3.

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+                              _
    |0 0 0 0|           ENSI (4 bits) [8 bits CNI]  |
    +-+-+-+-+-+-+-+-+                               } Encoding for
    |  LN1's Index  |   CNI  (8 bits)              _| Local Node LN1
    +-+-+-+-+-+-+-+-+                              _
    |0 0 1 1|           NN(4 bits)[3 adjacent nodes]|
    +-+-+-+-+                                       |
    |1 0 1|             E Flags [NN=3 bits]         | Encoding for
    +-+-+-+-+-+-+-+-+                               | 3 adjacent nodes
    |  RN1's Index  |   CNI (8 bits) for RN1        } (RN1, RN2, RN3)
    +-+-+-+-+-+-+-+-+                               | of LN1
    |  RN2's Index  |   CNI (8 bits) for RN2        | with E Flags
    +-+-+-+-+-+-+-+-+                               |
    |  RN3's Index  |   CNI (8 bits) for RN3       _|
    +-+-+-+-+-+-+-+-+                              _
    |0 0 0 1|           NN (4 bits)[1 adjacent node]|
    +-+-+-+-+                                       | Encoding for
    |0|                 E Flags [NN=1 bit]          } 1 adjacent node
    +-+-+-+-+-+-+-+-+                               | (RN11) of RN1
    |  RN11's Index |   CNI (8 bits) for RN11      _| with E Flags
    +-+-+-+-+-+-+-+-+                              _
    |0 0 1 0|           NN(4 bits)[2 adjacent nodes]|
    +-+-+-+-+                                       |
    |0 0|               E Flags [NN=2 bits]         | Encoding for
    +-+-+-+-+-+-+-+-+                               } 2 adjacent nodes
    |  RN31's Index |   CNI (8 bits) for RN31       | (RN31, RN32)
    +-+-+-+-+-+-+-+-+                               | of RN3 as a
    |  RN32's Index |   CNI (8 bits) for RN32       | local node
    +-+-+-+-+-+-+-+-+                              _| with E Flags

          Block Encoding for FT block
          from LN1 to RN1, RN2 and RN3, and to RN11, RN31 and RN32





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   The first E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to one, which indicates that the links between the first
   adjacent node RN1 as a local node and its adjacent nodes are included
   below.  In this example, 1 link between RN1 and its adjacent node
   RN11 is represented by the encoding for the adjacent node RN11 of RN1
   as a local node.  The encoding for 1 adjacent node RN11 consists of
   an NN field of 4 bits with value of 1, E Flags field of NN = 1 bits,
   and the index of adjacent node RN11.  The size of the index of RN11
   is the same as that of local node LN1 indicated by the ENSI in the
   encoding for local node LN1.

   The second E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to zero, which indicates that no links between the second
   adjacent node RN2 as a local node and its adjacent nodes are included
   below.

   The third E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to one, which indicates that the links between the third
   adjacent node RN3 as a local node and its adjacent nodes are included
   below.  In this example, 2 links between RN3 and its 2 adjacent nodes
   RN31 and RN32 are represented by the encoding for the adjacent nodes
   RN31 and RN32 of RN3 as a local node.  The encoding for 2 adjacent
   nodes RN31 and RN32 consists of an NN field of 4 bits with value of
   2, E Flags field of NN = 2 bits, and the indexes of adjacent nodes
   RN31 and RN32.  The size of the index of RN31 and RN32 is the same as
   that of local node LN1 indicated by the ENSI in the encoding for
   local node LN1.

   A new TLV is defined to contain a number of block encodings, and is
   called Flooding Topology Blocks TLV or Blocks TLV for short.  Its
   format is illustrated below.  This TLV may be used in the place of
   Links TLV in Section 6.2.1.1 for more efficiency.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      FTBK-TLV-Type (TBD5)     |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~     Block Encoding (for FT block from Node i to               ~
    ~                     its adjacent Nodes, and so on)            ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~     Block Encoding (for FT block from Node j to               ~
    ~                     its adjacent Nodes, and so on)            ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                        Flooding Topology Blocks TLV




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6.2.2.  Leaders Selection

   The leader or Designated Router (DR) selection for a broadcast link
   is about selecting two leaders: a DR and Backup DR.  This is
   generalized to select two or more leaders for an area: the primary/
   first leader (or leader for short), the secondary leader, the third
   leader and so on.

   A new TLV is defined to include the information on flooding reduction
   of a node, which is called Flooding Reduction Information TLV or
   Information TLV for short.  This TLV is generated by every node that
   supports flooding reduction in general.  Every node originates a RI
   LSA with a Flooding Reduction Information TLV containing its priority
   to become a leader.  The format of the TLV is as follows.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      INFO-TLV-Type (TBD6)     |          TLV-Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Priority   |             Reserved (MUST be zero)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Flooding Reduction Information TLV


   A Priority field of eight bits is defined in the TLV to indicate the
   priority of the node originating the TLV to become the leader node in
   centralized mode.

   A sub-TLV called leaders sub-TLV is defined.  It has the following
   format.

















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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      LEADS-TLV-Type (TBD7)    |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    1st Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    2nd Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    nth Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Leaders sub-TLV


   When a node selects itself as a leader, it originates a RI LSA
   containing the leader in a leaders sub-TLV.

   After the first leader node is down, the other leaders will be
   promoted.  The secondary leader becomes the first leader, the third
   leader becomes the secondary leader, and so on.  When a node selects
   itself as the n-th leader, it originates a RI LSA with a Leaders sub-
   TLV containing n leaders.

7.  Extensions to IS-IS

   The extensions to IS-IS is similar to OSPF.

7.1.  Extensions for Operations

   A new TLV for operations is defined in IS-IS LSP.  It has the
   following format and contains the same contents as the Flooding
   Reduction Instruction TLV defined in OSPF RI LSA.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |INS-Type(TBDi1)|    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OP | MOD |   Algorithm   |    Reserved (MUST be zero)  |  NL |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    IS-IS Flooding Reduction Instruction TLV




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7.2.  Extensions for Centralized Mode

7.2.1.  TLV for Flooding Topology

   A new TLV for the encodings of the links in the flooding topology is
   defined.  It has the following format and contains the same contents
   as the Flooding Topology Links TLV defined in OSPF Flooding Topology
   Opaque LSA.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |FTL-Type(TBDi2)|    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                      IS-IS Flooding Topology Links TLV


   Another new TLV for the encodings of the blocks in the flooding
   topology is defined.  It has the format below and contains the same
   contents as the Flooding Topology Blocks TLV defined in previous
   section.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |FTB-Type(TBDi3)|     Length    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~     Block Encoding (for FT block from Node i to               ~
    ~                     its adjacent Nodes, and so on)            ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~     Block Encoding (for FT block from Node j to               ~
    ~                     its adjacent Nodes, and so on)            ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                     ISIS Flooding Topology Blocks TLV









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7.2.2.  Leaders Selection

   Similar to Flooding Reduction Information TLV in OSPF, a new TLV
   called IS-IS Flooding Reduction Information TLV is defined.  It has
   the following format and contains the same contents as Flooding
   Reduction Information TLV in OSPF.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |INF-Type(TBDi4)|     Length    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Priority   |             Reserved (MUST be zero)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  IS-IS Flooding Reduction Information TLV


   A sub-TLV called IS-IS leaders sub-TLV is defined.  It has the
   following format and contains the contents similar to those in
   leaders sub-TLV in OSPF.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |LeadType(TBDi5)|    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    1st Leader Node/System ID                  ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    2nd Leader Node/System ID                  ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    nth Leader Node/System ID                  ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           IS-IS Leaders sub-TLV


8.  Flooding Behavior

   This section describes the revised flooding behavior for a node.  The
   revised flooding procedure MUST flood an LS to every node in the
   network in any case, as the standard flooding procedure does.





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8.1.  Nodes Perform Flooding Reduction without Failure

8.1.1.  Receiving an LS

   When a node receives a newer LS that is not originated by itself from
   one of its interfaces, it floods the LS only to all the other
   interfaces that are on the flooding topology.

   When the LS is received from an interface on the flooding topology,
   it is flooded only to all the other interfaces that are on the
   flooding topology.  When the LS is received on an interface that is
   not on the flooding topology, it is also flooded only to all the
   other interfaces that are on the flooding topology.

   In any case, the LS must not be transmitted back to the receiving
   interface.

   Note before forwarding a received LS, the node would do the normal
   processing as usual.

8.1.2.  Originating an LS

   When a node originates an LS, it floods the LS to its interfaces on
   the flooding topology if the LS is a refresh LS (i.e., there is no
   significant change in the LS comparing to the previous LS); otherwise
   (i.e., there are significant changes such as link down in the LS), it
   floods the LS to all its interfaces.  Choosing flooding the LS with
   significant changes to all the interfaces instead of limiting to the
   interfaces on the flooding topology would speed up the distribution
   of the significant link state changes.

8.1.3.  Establishing Adjacencies

   Adjacencies being established can be classified into two categories:
   adjacencies to new nodes and adjacencies to existing nodes.

8.1.3.1.  Adjacency to New Node

   An adjacency to a new node is an adjacency between an existing node
   (say node E) on the flooding topology and the new node (say node N)
   which is not on the flooding topology.  There is not any adjacency
   between node N and a node in the network area.  The procedure for
   establishing the adjacency between E and N is the existing normal
   procedure unchanged.

   When the adjacency between N and E is established, node E adds node N
   and the link between N and E to the flooding topology temporarily
   until a new flooding topology is built.  New node N adds node N and



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   the link between N and E to the flooding topology temporarily until a
   new flooding topology is built.

8.1.3.2.  Adjacency to Existing Node

   An adjacency to an existing node is an adjacency between two nodes
   (say nodes E and X) on the flooding topology.  The procedure for
   establishing the adjacency between E and X is the existing normal
   procedure unchanged.

   Both node E and node X assume that the link between E and X is not on
   the flooding topology until a new flooding topology is built.  After
   the adjacency between E and X is established, node E does not send
   node X any new or updated LS that it receives or originates, and node
   X does not send node E any new or updated LS that it receives or
   originates until a new flooding topology is built.

8.2.  An Exception Case

   During an LS flooding, one or more link and node failures may happen.
   Some failures do not split the flooding topology, thus do not affect
   the flooding behavior.  For example, multiple failures of the links
   not on the flooding topology do not split the flooding topology and
   do not affect the flooding behavior.  The sections below focus on the
   failures that may split the flooding topology.

8.2.1.  Multiple Failures

   When two or more failures on the current flooding topology occur
   almost in the same time, each of the nodes within a given distance
   (such as 3 hops) to a failure point, floods the link state (LS) that
   it receives or originates to all its links (except for the one from
   which the LS is received) until a new flooding topology is built.

   In other words, when the failures happen, each of the nodes within a
   given distance to a failure point, adds all its local links to the
   flooding topology temporarily until a new flooding topology is built.

   In alternative way, each node computes and maintains a small number
   of backup paths.  For a backup path for a link L on the flooding
   topology, a node N computes and maintains it only if the backup path
   goes through node N.  Node N stores the links (e.g., local link L1
   and L2) attached to it and on the backup path for link L.  When link
   L fails and there are one or more other failures on the flooding
   topology or the flooding topology splits, node N adds the links
   (e.g., L1 and L2) to the flooding topology temporarily until a new
   flooding topology is built.




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   Similarly, for a backup path for a connection crossing a node M on
   the flooding topology, a node N computes and maintains it only if the
   backup path goes through node N.  Node N stores the links (e.g.,
   local link La and Lb) attached to it and on the backup path for node
   M.

   When node M fails and there are one or more other failures on the
   flooding topology or the flooding topology splits, node N adds the
   links (e.g., La and Lb) to the flooding topology temporarily until a
   new flooding topology is built.

   For one link/node failure that splits the current flooding topology,
   the above behavior is applied.

   Note that if it can be quickly determined that the flooding topology
   is not split by the failures, the flooding behavior in Section 8.1
   may follow.

8.2.2.  Changes on Flooding Topology

   After multiple failures split the current (old) flooding topology,
   some link states may be out of synchronization among some nodes.
   This can be resolved as follows.

   After a node N computes or receives a new flooding topology, for a
   local link L attached to node N, if 1) link L is not on the current
   (old) flooding topology and is on the new flooding topology, and 2)
   there is a failure after the current (old) flooding topology is
   built, then node N sends a delta of the link states that it received
   or originated to its adjacent node over link L.

   For node N, the delta of the link states is the link states with
   changes that node N received or originated during the period of time
   in which the current (old) flooding topology is split.

   Suppose that Max_Split_Period is a number (in seconds), which is the
   maximum period of time in which a flooding topology is split.  Tc is
   the time at which the current (old) flooding topology is built, Tn is
   the time at which the new flooding topology is built, and Ts is the
   bigger one between Tc and (Tn - Max_Split_Period).  Node N sends its
   adjacent node over link L the link states with changes that it
   received or originated from Ts to Tn.

9.  Operations on Flooding Reduction







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9.1.  Configuring Flooding Reduction

   This section describes configurations for link state flooding
   reduction, including configurations for centralized flooding
   reduction (i.e., flooding reduction in centralized mode) and
   configurations for distributed flooding reduction (i.e., flooding
   reduction in distributed mode).

9.1.1.  Configurations for Centralized Flooding Reduction

   At first, for each node, if it is eligible to become a leader for
   flooding reduction in centralized mode, a user configures a priority
   on the node for the leader election.  The value range for the
   priority is from 0 to 255.  A node with a priority set to zero cannot
   become a leader.  The node with the higher priority has the higher
   precedence to be elected as the leader.

   And then, a user selects the centralized mode on one node, which
   tells the other nodes in the area to use centralized flooding
   reduction.

9.1.2.  Configurations for Distributed Flooding Reduction

   For distributed flooding reduction, an algorithm for computing a
   flooding topology needs to be configured.  The algorithm and
   distributed mode are configured on one node, which tells the other
   nodes in the area the algorithm and the mode via advertising the
   number of the algorithm and the mode.  Every node participating in
   the distributed flooding reduction uses this same algorithm.

9.2.  Migration to Flooding Reduction

   Migrating a OSPF or IS-IS area from normal flooding to flooding
   reduction smoothly takes a few steps or stages.  This section
   describes the steps for migrating an area to centralized flooding
   reduction or distributed flooding reduction from normal flooding.

9.2.1.  Migration to Centralized Flooding Reduction

   At first, a user configures a priority on every node that is eligible
   for the leader for centralized flooding reduction.  In this stage, a
   node does not originate or advertise its priority.

   Second, after configuring the priority, a user selects the
   centralized mode on one node, which tells the other nodes in the area
   to use centralized flooding reduction.





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   After a node knows that the centralized mode is used, it originates
   and advertises its priority.  The leader election is started in the
   area.  A user may check whether a leader is elected through showing
   the link state containing leaders.  After the leader is elected, the
   centralized flooding reduction may be activated.

   And then, a user activates the flooding reduction through using a
   configuration such as perform flooding Reduction on one node, which
   tells all the nodes in the area to use centralized flooding
   reduction.  The node generates and advertises a link state with OP =
   R (indicating perform flooding Reduction) after it receives the
   configuration.  After another node in the area receives the link
   state with OP = R, it also perform flooding reduction (i.e., floods
   link states using flooding topology).  Thus, activating the flooding
   reduction on one node propagates to every node in the area, which
   migrates to flooding reduction.

9.2.2.  Migration to Distributed Flooding Reduction

   At first, a user selects the distributed mode on one node, which
   tells the other nodes in the area to use distributed flooding
   reduction.

   After a node knows that the distributed mode is used, it advertises
   the algorithms it supports.  A user may check whether every node
   advertises its supporting algorithms through showing the link state
   containing the algorithms.

   And then, a user selects an algorithm and activates the flooding
   reduction through using configurations such as perform flooding
   Reduction on one node, which tells all the nodes in the area to use
   the given algorithm and start the distributed flooding reduction.

9.3.  Roll Back to Normal Flooding

   For rolling back from flooding reduction to normal flooding, a user
   de-activates the flooding reduction through configuring roll back to
   normal flooding on one node, which tells all the nodes in the area to
   roll back to normal flooding.

   After receiving a configuration to roll back to normal flooding, the
   node floods link states using all its local links instead of the
   local links on the flooding topology.  It also advertises the roll
   back to Normal flooding (i.e., OP = N) to all the other nodes in the
   area.  When each of the other nodes receives the advertisement, it
   rolls back to normal flooding (i.e., floods link states using all its
   local links instead of the local links on the flooding topology).




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   In centralized mode, after rolling back to normal flooding, the
   leader of the area stops computing and advertising a flooding
   topology, the other nodes stops receiving and building the flooding
   topology.  In distributed mode, every node in the area will not
   compute or build flooding topology.

9.4.  Transfer from Distributed to Centralized Mode

   When the distributed flooding reduction in an area is running, in
   order to transfer it to centralized flooding reduction, a user may
   take the following steps.

   At first, the user rolls back from flooding reduction to normal
   flooding as described in section "Roll Back to Normal Flooding".

   And then, the user migrates to centralized flooding reduction from
   normal flooding as described in section "Migration to Centralized
   Flooding Reduction".

   Alternatively, the user may just change the flooding reduction mode
   from distributed mode to centralized mode on one node through a
   configuration.  After receiving the configuration for changing the
   mode, the node transfers from distributed mode to centralized mode
   and tells the other nodes the change through advertising MOD = C
   (i.e., Centralized mode).  After receiving the advertisement, each of
   the other nodes transfers from distributed mode to centralized mode.

   Note that before changing the flooding reduction mode to centralized
   mode, the user needs to configure a priority on every node that is
   eligible for the leader for centralized flooding reduction.

   While transferring from distributed mode to centralized mode, a node
   uses the distributed flooding reduction (i.e., floods the link states
   over its local links on the flooding topology computed and built by
   itself) until the centralized flooding reduction is fully functional
   for a given time such as 5 seconds.  After this time, the node stops
   its distributed flooding reduction, i.e., stops computing and
   building its flooding topology, and using this flooding topology to
   flood the link states.

   Each node in the area advertises its priority.  A leader will be
   elected for the area.  The leader starts to compute a flooding
   topology and floods it to all the other nodes.  Every node builds the
   flooding topology computed by the leader and starts to flood the link
   states over its local links on this flooding topology.






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9.5.  Transfer from Centralized to Distributed Mode

   When the centralized flooding reduction in an area is running, in
   order to transfer it to distributed flooding reduction, a user may
   take the following steps.

   At first, the user rolls back from flooding reduction to normal
   flooding as described in section "Roll Back to Normal Flooding".

   And then, the user migrates to distributed flooding reduction from
   normal flooding as described in section "Migration to Distributed
   Flooding Reduction".

   Alternatively, the user may just change the flooding reduction mode
   from centralized mode to distributed mode on one node through a
   configuration.  After receiving the configuration for changing the
   mode, the node transfers from centralized mode to distributed mode
   and tells the other nodes the change through advertising MOD = D
   (i.e., Distributed mode).  After receiving the advertisement, each of
   the other nodes transfers from centralized mode to distributed mode.

   While transferring from centralized mode to distributed mode, a node
   uses the centralized flooding reduction (i.e., floods the link states
   over its local links on the flooding topology computed by the leader
   of the area) until the distributed flooding reduction is fully
   functional for a given time.  After this time, the node stops its
   centralized flooding reduction.  The leader stops computing the
   flooding topology, advertising it to all the other routers, and using
   this flooding topology to flood the link states.  Each of the other
   nodes stops receiving and building the flooding topology computed by
   the leader.

   Every node starts to compute and build its flooding topology and to
   flood the link states over its local links on this flooding topology.

9.6.  Adding a New Node to Network

   If the centralized flooding reduction is used in an area, for adding
   a new node (say node N) to the area, a user configures a priority for
   this new node to become the leader of the area.

   The other configurations on the new node are the existing normal ones
   unchanged.

   When the new node N is connected via a link to a node (say E) on the
   flooding topology, there is not any adjacency between them (i.e., N
   and E) over the link.  The procedure for establishing the adjacency
   between N and E is the existing normal procedure unchanged.



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   Node E adds node N and the link between N and E to the flooding
   topology temporarily until a new flooding topology is built.

   New node N adds node N and the link between N and E to the flooding
   topology temporarily until a new flooding topology is built.

10.  Manageability Considerations

   Section 9 "Operations on Flooding Reduction" outlines the
   configuration process and deployment scenarios for link state
   flooding reduction.  The configurable items include to set the
   priority of a node to become a leader of the area for link state
   flooding reduction in centralized mode.  The flooding reduction
   function may be controlled by a policy module and assigned a suitable
   user privilege level to enable.  A suitable model may be required to
   verify the flooding reduction status on routers participating in the
   flooding reduction, including their role as a leader in centralized
   mode or a normal node advertising link states using flooding
   topology.  The mechanisms defined in this document do not imply any
   new liveness detection and monitoring requirements in addition to
   those indicated in [RFC2328] and [RFC1195].

11.  Security Considerations

   A notable beneficial security aspect of link state flooding reduction
   is that the flooding topology in the centralized mode is advertised
   in a single area, and a link state is not advertised over every link,
   but over the links on the flooding topology.  It should be noted that
   a malicious node could inject a fake flooding topology in the
   centralized mode, which could lead inconsistent link state databases
   among the nodes in an area.  The malicious node could inject a link
   state with the OP field set to R or N, which could trigger the
   migration or roll back into/from a flooding reduction.  Good security
   practice might reuse the IS-IS authentication in [RFC5304] as well as
   [RFC5310], and the OSPF authentication and other security mechanisms
   described in [RFC2328], [RFC4552] and [RFC7474] to mitigate this type
   of risk.

12.  IANA Considerations

12.1.  OSPFv2

   Under Registry Name: OSPF Router Information (RI) TLVs [RFC7770],
   IANA is requested to assign two new TLV values for OSPF flooding
   reduction as follows:






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     +===============+==================+=====================+
     |  TLV Value    |    TLV Name      |    reference        |
     +===============+==================+=====================+
     |      11       | Instruction TLV  |    This document    |
     +---------------+------------------+---------------------+
     |      12       | Information TLV  |    This document    |
     +---------------+------------------+---------------------+

   Under the registry name "Opaque Link-State Advertisements (LSA)
   Option Types" [RFC5250], IANA is requested to assign new Opaque Type
   registry values for FT LSA as follows:

     +====================+===============+=======================+
     |  Registry Value    |  Opaque Type  |    reference          |
     +====================+===============+=======================+
     |         10         |    FT LSA     |    This document      |
     +--------------------+---------------+-----------------------+

   IANA is requested to create and maintain new registries:

    o OSPFv2 FT LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv2 FT LSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             FT Links TLV               see Section 6.2.1.1
       2             FT Blocks TLV              see Section 6.2.1.2
       3-32767       Unassigned
       32768-65535   Reserved

12.2.  OSPFv3

   Under the registry name "OSPFv3 LSA Function Codes", IANA is
   requested to assign new registry values for FT LSA as follows:

     +===========+==========================+=======================+
     |  Value    |  LSA Function Code Name  |    reference          |
     +======================================+=======================+
     |    16     |         FT LSA           |    This document      |
     +-----------+--------------------------+-----------------------+

   IANA is requested to create and maintain new registries:

    o OSPFv3 FT LSA TLVs




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   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv3 FT LSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             FT Links TLV               see Section 6.2.1.1
       2             FT Blocks TLV              see Section 6.2.1.2
       3-32767       Unassigned
       32768-65535   Reserved

12.3.  IS-IS

   Under Registry Name: IS-IS TLV Codepoints, IANA is requested to
   assign new TLV values for IS-IS flooding reduction as follows:

       Value     TLV Name                      Definition
       -----    ------------------------       ----------
        151      FT Links TLV                   see Section 7.2.1
        152      FT Blocks TLV                  see Section 7.2.1
        153      Instruction TLV                see Section 7.1
        154      Information TLV                see Section 7.2.2

13.  Acknowledgements

   The authors would like to thank Acee Lindem, Zhibo Hu, Robin Li,
   Stephane Litkowski and Alvaro Retana for their valuable suggestions
   and comments on this draft.

14.  References

14.1.  Normative References

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.






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   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
              OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
              July 2008, <https://www.rfc-editor.org/info/rfc5250>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

14.2.  Informative References

   [I-D.li-dynamic-flooding]
              Li, T. and P. Psenak, "Dynamic Flooding on Dense Graphs",
              draft-li-dynamic-flooding-05 (work in progress), June
              2018.

   [I-D.shen-isis-spine-leaf-ext]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-07 (work in progress), October 2018.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.




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Appendix A.  Algorithms to Build Flooding Topology

   There are many algorithms to build a flooding topology.  A simple and
   efficient one is briefed below.

   o  Select a node R according to a rule such as the node with the
      biggest/smallest node ID;

   o  Build a tree using R as root of the tree (details below); and then

   o  Connect k (k>=0) leaves to the tree to have a flooding topology
      (details follow).

A.1.  Algorithms to Build Tree without Considering Others

   An algorithm for building a tree from node R as root starts with a
   candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.

   4.  Add Xi (i = 1, 2, ..., n) into the end of Cq, goto step 1.

   Another algorithm for building a tree from node R as root starts with
   a candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.



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   4.  Add Xi (i = 1, 2, ..., n) into the front of Cq and goto step 1.

   A third algorithm for building a tree from node R as root starts with
   a candidate list Cq containing R associated with cost 0 and an empty
   flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, Cai is the cost associated with Ai,
       and IDi is the ID of Ai.  One order is that for any k = 1, 2,
       ..., m-1, Cak < Caj (j = k+1) or Cak = Caj and IDk < IDj.  Goto
       step 1.

A.2.  Algorithms to Build Tree Considering Others

   An algorithm for building a tree from node R as root with
   consideration of others's support for flooding reduction starts with
   a candidate queue Cq containing R associated with previous hop PH=0
   and an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them that support
       flooding reduction (.  For example, X1, X2, ..., Xn are ordered
       by the cost of the link between A and Xi.  The cost of the link
       between A and Xi is less than that of the link between A and Xj
       (j = i + 1).  If two costs are the same, Xi's ID is less than
       Xj's ID.  The cost of a link is redefined such that 1) the cost
       of a link between A and Xi both support flooding reduction is



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       much less than the cost of any link between A and Xk where Xk
       with F=0; 2) the real metric of a link between A and Xi and the
       real metric of a link between A and Xk are used as their costs
       for determining the order of Xi and Xk if they all (i.e., A, Xi
       and Xk) support flooding reduction or none of Xi and Xk support
       flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the end of the candidate queue Cq, and goto step 1.

   Another algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction starts with a
   candidate queue Cq containing R associated with previous hop PH=0 and
   an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft.

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them support flooding
       reduction.  For example, X1, X2, ..., Xn are ordered by the cost
       of the link between A and Xi.  The cost of the link between A and
       Xi is less than the cost of the link between A and Xj (j = i +
       1).  If two costs are the same, Xi's ID is less than Xj's ID.
       The cost of a link is redefined such that 1) the cost of a link
       between A and Xi both support flooding reduction is much less
       than the cost of any link between A and Xk where Xk does not
       support flooding reduction; 2) the real metric of a link between
       A and Xi and the real metric of a link between A and Xk are used
       as their costs for determining the order of Xi and Xk if they all
       (i.e., A, Xi and Xk) support flooding reduction or none of Xi and
       Xk supports flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the front of the candidate queue Cq, and goto step 1.

   A third algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction (using flag F
   = 1 for support, and F = 0 for not support in the following) starts
   with a candidate list Cq containing R associated with low order cost
   Lc=0, high order cost Hc=0 and previous hop ID PH=0, and an empty
   flooding topology Ft:




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   1.  Remove the first node A from Cq and add A into Ft.

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Suppose that node A is associated with a low order cost LCa which
       is the low order cost from root R to node A and a high order cost
       HCa which is the high order cost from R to A, node Xi (i = 1, 2,
       ..., n) is connected to node A and not in the flooding topology
       Ft and the real cost of the link between A and Xi is Ci (i=1, 2,
       ..., n).  Compute LCxi and HCxi: LCxi = LCa + Ci if both A and Xi
       have flag F set to one, otherwise LCxi = LCa HCxi = HCa + Ci if A
       or Xi does not have flag F set to one, otherwise HCxi = HCa If Xi
       is not in Cq, then add Xi associated with LCxi, HCxi and PH = A
       into Cq; If Xi associated with LCxi' and HCxi' and PHxi' is in
       Cq, then If HCxi' > HCxi then replace Xi with HCxi', LCxi' and
       PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise (i.e.,
       HCxi' == HCxi) if LCxi' > LCxi , then replace Xi with HCxi',
       LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise
       (i.e., HCxi' == HCxi and LCxi' == LCxi) if PHxi' > PH, then
       replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and
       PH=A in Cq.

   5.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, HCai and LCai are low order cost and
       high order cost associated with Ai, and IDi is the ID of Ai.  One
       order is that for any k = 1, 2, ..., m-1, HCak < HCaj (j = k+1)
       or HCak = HCaj and LCak < LCaj or HCak = HCaj and LCak = LCaj and
       IDk < IDj.  Goto step 1.

A.3.  Connecting Leaves

   Suppose that we have a flooding topology Ft built by one of the
   algorithms described above.  Ft is like a tree.  We may connect k (k
   >=0) leaves to the tree to have a enhanced flooding topology with
   more connectivity.

   Suppose that there are m (0 < m) leaves directly connected to a node
   X on the flooding topology Ft.  Select k (k <= m) leaves through
   using a deterministic algorithm or rule.  One algorithm or rule is to



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   select k leaves that have smaller or larger IDs (i.e., the IDs of
   these k leaves are smaller/bigger than the IDs of the other leaves
   directly connected to node X).  Since every node has a unique ID,
   selecting k leaves with smaller or larger IDs is deterministic.

   If k = 1, the leaf selected has the smallest/largest node ID among
   the IDs of all the leaves directly connected to node X.

   For a selected leaf L directly connected to a node N in the flooding
   topology Ft, select a connection/adjacency to another node from node
   L in Ft through using a deterministic algorithm or rule.

   Suppose that leaf node L is directly connected to nodes Ni (i =
   1,2,...,s) in the flooding topology Ft via adjacencies and node Ni is
   not node N, IDi is the ID of node Ni, and Hi (i = 1,2,...,s) is the
   number of hops from node L to node Ni in the flooding topology Ft.

   One Algorithm or rule is to select the connection to node Nj (1 <= j
   <= s) such that Hj is the largest among H1, H2, ..., Hs.  If there is
   another node Na ( 1 <= a <= s) and Hj = Ha, then select the one with
   smaller (or larger) node ID.  That is that if Hj == Ha and IDj < IDa
   then select the connection to Nj for selecting the one with smaller
   node ID (or if Hj == Ha and IDj < IDa then select the connection to
   Na for selecting the one with larger node ID).

   Suppose that the number of connections in total between leaves
   selected and the nodes in the flooding topology Ft to be added is
   NLc.  We may have a limit to NLc.

Authors' Addresses

   Huaimo Chen
   Huawei Technologies
   Boston
   USA

   Email: huaimo.chen@huawei.com


   Dean Cheng
   Huawei Technologies
   Santa Clara
   USA

   Email: dean.cheng@huawei.com






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   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com


   Yi Yang
   IBM
   Cary, NC
   United States of America

   Email: yyietf@gmail.com


   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209
   China

   Email: wangaj.bri@chinatelecom.cn


   Xufeng Liu
   Volta Networks
   McLean, VA
   USA

   Email: xufeng.liu.ietf@gmail.com


   Yanhe Fan
   Casa Systems
   USA

   Email: yfan@casa-systems.com


   Lei Liu
   USA

   Email: liulei.kddi@gmail.com








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